U.S. patent number 8,071,958 [Application Number 12/268,524] was granted by the patent office on 2011-12-06 for ion implantation device and a method of semiconductor manufacturing by the implantation of boron hydride cluster ions.
This patent grant is currently assigned to SemEquip, Inc.. Invention is credited to Thomas N. Horsky, Dale C. Jacobson.
United States Patent |
8,071,958 |
Horsky , et al. |
December 6, 2011 |
Ion implantation device and a method of semiconductor manufacturing
by the implantation of boron hydride cluster ions
Abstract
A method of manufacturing a semiconductor device includes the
steps of: providing a supply of molecules containing a plurality of
dopant atoms into an ionization chamber, ionizing said molecules
into dopant cluster ions, extracting and accelerating the dopant
cluster ions with an electric field, selecting the desired cluster
ions by mass analysis, modifying the final implant energy of the
cluster ion through post-analysis ion optics, and implanting the
dopant cluster ions into a semiconductor substrate. In general,
dopant molecules contain n dopant atoms, where n is an integer
number greater than 10. This method enables increasing the dopant
dose rate to n times the implantation current with an equivalent
per dopant atom energy of 1/n times the cluster implantation
energy, while reducing the charge per dopant atom by the factor
n.
Inventors: |
Horsky; Thomas N. (Boxborough,
MA), Jacobson; Dale C. (Salem, NH) |
Assignee: |
SemEquip, Inc. (North
Billerica, MA)
|
Family
ID: |
29779196 |
Appl.
No.: |
12/268,524 |
Filed: |
November 11, 2008 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090090872 A1 |
Apr 9, 2009 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
10519999 |
|
|
|
|
|
PCT/US03/20197 |
Jun 26, 2003 |
|
|
|
|
10183768 |
Jun 26, 2002 |
6686595 |
|
|
|
60463965 |
Apr 18, 2003 |
|
|
|
|
Current U.S.
Class: |
250/427;
250/492.2; 315/111.81; 118/723CB; 250/423R; 250/492.21; 118/723R;
315/111.91; 250/425 |
Current CPC
Class: |
H01L
29/66575 (20130101); H01J 37/08 (20130101); H01L
27/092 (20130101); H01L 21/2658 (20130101); H01L
21/26513 (20130101); H01L 21/823814 (20130101); H01J
27/20 (20130101); H01L 21/823842 (20130101); H01J
37/3171 (20130101); H01L 21/67213 (20130101); H01L
21/3215 (20130101); H01J 2237/061 (20130101); H01J
2237/082 (20130101); H01J 2237/304 (20130101) |
Current International
Class: |
H01J
49/10 (20060101); H01J 27/02 (20060101); H01J
49/14 (20060101) |
Field of
Search: |
;250/427,423R,425,492.21,492.2 ;315/111.81,111.91
;118/723CB,723R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1119347 |
|
Mar 1996 |
|
CN |
|
1258125 |
|
Dec 1971 |
|
GB |
|
Other References
Goto et al. "Novel Shallow Junction Technology Using Decarborane
(B1OH14)" Electron Devices Meeting, 1996, Int'l San Fancisco, CA,
USA Dec. 8-11, 1996, NY, NY, USA, IEEE, US, p. 435-438,
XP010207579. cited by other .
Ishikawa et al.: "Negative-Ion Implantation Technique" Nuclear
Instruments & Methods in Physics Research, Section--B: Beam
Interactions With Materials and Atoms, Elsevier, Amsterdam, NL,
vol. 96, No. 1/2, Mar. 1995, pp. 7-12, XP004010949. cited by other
.
Takeuchi et al. "Shallow Junction Formation by Polyatomic Cluster
Ion Implantation," Austin, TX, US Jun. 16-21, 1996, NY, NY USA,
IEEE, US p. 772-775, XP010220650. cited by other .
Boggia et al. Study of a Trapped Ion source, IEEE Journal, pp.
1433-1435, Jun. 1998. cited by other .
Brautti et al., Trapped Ion source, IEEE Journal, 1988, pp.
2729-2731. cited by other .
Chenglu et al., Nuclear Instruments and Methods in Physics
Research, 1989, pp. 384-386. cited by other .
Defino et al., J. electrochemical Society, vol. 133, No. 9, 1986,
pp. 1900-1904. cited by other .
Jacobson et al., "Decarborane, an Alternative Approach to Ultra Low
Energy Ion Implantation," IEEE Journal, 2000, pp. 300-303. cited by
other .
Kishimoto, "A High-current Negative-Ion Implanter and its
Application for Nanocrystal Fabrication in Insulators," IEEE
Journal, 1999, pp. 342-345. cited by other .
Olsen et al., J. Amer. Chem. Society, vol. 90, No. 15, Jul. 17,
1986, pp. 3946-3951. cited by other .
Tsubouchi et al., "Beam Characterization of Mass-Separated,
Low-energy Positive and Negative Ions Deposition Apparatus," IEEE
Proc. of the X11th Int'l Conf. on Ion Implantation Technology, Jun.
22-26, 1998, pp. 350-353. cited by other .
Yamada, Applications of Gas Cluster Ion Beams for Materials
Processing, Oct. 30, 1997. cited by other .
EPO Search Report 03762087, Jan. 17, 2008. cited by other.
|
Primary Examiner: Wells; Nikita
Attorney, Agent or Firm: Paniaguas; John S. Katten Muchin
Rosenman LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a division of U.S. application Ser. No.
10/519,699, filed on Sep. 14, 2005, which is a national stage
application under 35 USC .sctn.371 of International Application No.
PCT/US03/20197, filed on Jun. 26, 2003, which is a
continuation-in-part of U.S. patent application Ser. No.
10/183,768, now U.S. Pat. No. 6,686,595. This application also
claims priority and the benefit of U.S. Provisional Patent
Application No. 60/463,965, filed on Apr. 18, 2003, entitled "An
Ion Implantation Device and Method of Semiconductor Manufacturing
by the Implantation of Boron Hydride Cluster Ions" and U.S.
application Ser. No. 10/183,768, filed on Jun. 26, 2002, entitled
Electron Impact Ion Source.
Claims
What is claimed and desired to be covered by a Letters Patent is as
follows:
1. An ion source comprising: a source of gas; an ionization chamber
in fluid communication with said source of gas, said ionization
chamber formed with one or more electron entrance apertures, for
receiving one or more electron, beams generated by an ion source
located outside of said ionization chamber, an ion extraction
aperture for enabling an ionized beam to be extracted and a gas
inlet aperture, said ionization chamber configured to enable
ionization of said gas by electron bombardment; one or more
electron sources for generating one or more electron beams, said
electron source disposed outside of said ionization chamber; and a
first source of magnetic flux for generating a magnetic field
within said ionization chamber, said source including a magnetic
yoke assembly disposed outside of said ionization chamber.
2. The ion source as recited in claim 1, wherein said magnetic yoke
assembly includes a permanent magnet.
3. The ion source as recited in claim 1, wherein said magnetic yoke
assembly includes an electromagnet.
4. The ion source as recited in claim 1, wherein said one or more
electron sources are configured so that said electron beam is
generally parallel to the plane or planes containing the one or
more electron entrance apertures, further including one or more
beam steerers for bending said one or more electron beams so as to
be generally perpendicular to the plane or planes containing said
one or more electron entrance apertures.
5. The ion source as recited in claim 1, wherein each of said one
or more beam steerers includes a second source of magnetic
flux.
6. The ion source of claim 1, further including a magnetic shield
between said electron sources and said magnetic yoke assembly, to
substantially prevent the field produced by said magnetic yoke
assembly from penetrating into the region of said electron sources.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of semiconductor
manufacturing in which P-type doping is accomplished by the
implantation of ion beams formed from ionized boron hydride
molecules, said ions being of the form B.sub.nH.sub.x.sup.+ and
B.sub.nH.sub.x.sup.-, where 10.ltoreq.n.ltoreq.100 and
0.ltoreq.x.ltoreq.n+4.
2. Description of the Prior Art
The Ion Implantation Process
The fabrication of semiconductor devices involves, in part, the
introduction of impurities into the semiconductor substrate to form
doped regions. The impurity elements are selected to bond
appropriately with the semiconductor material so as to create
electrical carriers, thus altering the electrical conductivity of
the semiconductor material. The electrical carriers can either be
electrons (generated by N-type dopants) or holes (generated by
P-type dopants). The concentration of dopant impurities so
introduced determines the electrical conductivity of the resultant
region. Many such N- and P-type impurity regions must be created to
form transistor structures, isolation structures and other such
electronic structures, which function collectively as a
semiconductor device.
The conventional method of introducing dopants into a semiconductor
substrate is by ion implantation. In ion implantation, a feed
material containing the desired element is introduced into an ion
source and energy is introduced to ionize the feed material,
creating ions which contain the dopant element (for example, in
silicon the elements .sup.75As, .sup.31P, and .sup.121Sb are donors
or N-type dopants, while .sup.11B and .sup.115In are acceptors or
P-type dopants). An accelerating electric field is provided to
extract and accelerate the typically positively-charged ions, thus
creating an ion beam (in certain cases, negatively-charged ions may
be used instead). Then, mass analysis is used to select the species
to be implanted, as is known in the art, and the mass-analyzed ion
beam may subsequently pass through ion optics which alter its final
velocity or change its spatial distribution prior to being directed
into a semiconductor substrate or workpiece. The accelerated ions
possess a well-defined kinetic energy which allows the ions to
penetrate the target to a well-defined, predetermined depth at each
energy value. Both the energy and mass of the ions determine their
depth of penetration into the target, with higher energy and/or
lower mass ions allowing deeper penetration into the target due to
their greater velocity. The ion implantation system is constructed
to carefully control the critical variables in the implantation
process, such as the ion energy, ion mass, ion beam current
(electrical charge per unit time), and ion dose at the target
(total number of ions per unit area that penetrate into the
target). Further, beam angular divergence (the variation in the
angles at which the ions strike the substrate) and beam spatial
uniformity and extent must also be controlled in order to preserve
semiconductor device yields.
A key process of semiconductor manufacturing is the creation of P-N
junctions within the semiconductor substrate. This requires the
formation of adjacent regions of P-type and N-type doping. An
important example of the formation of such a junction is the
implantation of P-type dopant into a semiconductor region already
containing a uniform distribution of N-type dopant. In this case,
an important parameter is the junction depth, which is defined as
the depth from the semiconductor surface at which the P-type and
N-type dopants have equal concentrations. This junction depth is a
function of the implanted dopant mass, energy and dose.
An important aspect of modern semiconductor technology is the
continuous evolution to smaller and faster devices. This process is
called scaling. Scaling is driven by continuous advances in
lithographic process methods, allowing the definition of smaller
and smaller features in the semiconductor substrate which contains
the integrated circuits. A generally accepted scaling theory has
been developed to guide chip manufacturers in the appropriate
resize of all aspects of the semiconductor device design at the
same time, i.e., at each technology or scaling node. The greatest
impact of scaling on ion implantation process is the scaling of
junction depths, which requires increasingly shallow junctions as
the device dimensions are decreased. This requirement for
increasingly shallow junctions as integrated circuit technology
scales translates into the following requirement: ion implantation
energies must be reduced with each scaling step. The extremely
shallow junctions called for by modern, sub-0.13 micron devices are
termed "Ultra-Shallow Junctions", or USJ.
Physical Limitations on Low-Energy Beam Transport
Due to the aggressive scaling of junction depths in CMOS
processing, the ion energy required for many critical implants has
decreased to the point that conventional ion implantation systems,
originally developed to generate much higher energy beams, deliver
much reduced ion currents to the wafer, reducing wafer throughput.
The limitations of conventional ion implantation systems at low
beam energy are most evident in the extraction of ions from the ion
source, and their subsequent transport through the implanter's beam
line. Ion extraction is governed by the Child-Langmuir relation,
which states that the extracted beam current density is
proportional to the extraction voltage (i.e., beam energy at
extraction) raised to the 3/2 power. FIG. 2 is a graph of maximum
extracted boron beam current versus extraction voltage. For
simplicity, an assumption has been made that only .sup.11B.sup.+
ions are present in the extracted beam. FIG. 2 shows that as the
energy is reduced, extraction current drops quickly. In a
conventional ion implanter, this regime of "extraction-limited"
operation is seen at energies less than about 10 keV. Similar
constraints affect the transport of the low-energy beam after
extraction. A lower energy ion beam travels with a smaller
velocity, hence for a given value of beam current the ions are
closer together, i.e., the ion density increases. This can be seen
from the relation J=.eta.ev, where J is the ion beam current
density in mA/cm.sup.2, .eta. is the ion density in ions/cm.sup.-3,
e is the electronic charge (=6.02.times.10.sup.-19 Coulombs), and v
is the average ion velocity in cm/s. In addition, since the
electrostatic forces between ions are inversely proportional to the
square of the distance between them, electrostatic repulsion is
much stronger at low energy, resulting in increased dispersion of
the ion beam. This phenomenon is called "beam blow-up", and is the
principal cause of beam loss in low-energy transport. While
low-energy electrons present in the implanter beam line tend to be
trapped by the positively-charged ion beam, compensating for
space-charge blow-up during transport, blow-up nevertheless still
occurs, and is most pronounced in the presence of electrostatic
focusing lenses, which tend to strip the loosely-bound, highly
mobile compensating electrons from the beam. In particular, severe
extraction and transport difficulties exist for light ions, such as
the P-type dopant boron, whose mass is only 11 amu. Being light,
boron atoms penetrate further into the substrate than other atoms,
hence the required implantation energies for boron are lower than
for the other implant species. In fact, extremely low implantation
energies of less than 1 keV are being required for certain leading
edge USJ processes. In reality, most of the ions extracted and
transported from a typical BF.sub.3 source plasma are not the
desired ion .sup.11B.sup.+, but rather ion fragments such as
.sup.19F.sup.+ and .sup.49BF.sub.2.sup.+; these serve to increase
the charge density and average mass of the extracted ion beam,
further increasing space-charge blow-up. For a given beam energy,
increased mass results in a greater beam perveance; since heavier
ions move more slowly, the ion density .eta. increases for a given
beam current, increasing space charge effects in accordance with
the discussion above.
Molecular Ion Implantation
One way to overcome the limitations imposed by the Child-Langmuir
relation discussed above is to increase the transport energy of the
dopant ion by ionizing a molecule containing the dopant of
interest, rather than a single dopant atom. In this way, while the
kinetic energy of the molecule is higher during transport, upon
entering the substrate, the molecule breaks up into its constituent
atoms, sharing the energy of the molecule among the individual
atoms according to their distribution in mass, so that the dopant
atom's implantation energy is much lower than the original
transport kinetic energy of the molecular ion. Consider the dopant
atom "X" bound to a radical "Y" (disregarding for purposes of
discussion the issue of whether "Y" affects the device-forming
process). If the ion XY.sup.+ were implanted in lieu of X.sup.+,
then XY.sup.+ must be extracted and transported at a higher energy,
increased by a factor equal to the mass of XY divided by the mass
of X; this ensures that the velocity of X in either case is the
same. Since the space-charge effects described by the
Child-Langmuir relation discussed above are super-linear with
respect to ion energy, the maximum transportable ion current is
increased. Historically, the use of polyatomic molecules to
ameliorate the problems of low energy implantation is well known in
the art. A common example has been the use of the BF.sub.2.sup.+
molecular ion for the implantation of low-energy boron, in lieu of
B.sup.+. This process dissociates BF.sub.3 feed gas to the
BF.sub.2.sup.+ ion for implantation. In this way, the ion mass is
increased to 49 AMU, allowing an increase of the extraction and
transport energy by more than a factor of 4 (i.e., 49/11) over
using single boron atoms. Upon implantation, however, the boron
energy is reduced by the same factor of (49/11). It is worthy of
note that this approach does not reduce the current density in the
beam, since there is only one boron atom per unit charge in the
beam. In addition, this process also implants fluorine atoms into
the semiconductor substrate along with the boron, an undesirable
feature of this technique since fluorine has been known to exhibit
adverse effects on the semiconductor device.
Cluster Implantation
In principle, a more effective way to increase dose rate than by
the XY.sup.+ model discussed above is to implant clusters of dopant
atoms, that is, molecular ions of the form X.sub.nY.sub.m.sup.+,
where n and m are integers and n is greater than one. Recently,
there has been seminal work using decaborane as a feed material for
ion implantation. The implanted particle was a positive ion of the
decaborane molecule, B.sub.10H.sub.14, which contains 10 boron
atoms, and is therefore a "cluster" of boron atoms. This technique
not only increases the mass of the ion and hence the transport ion
energy, but for a given ion current, it substantially increases the
implanted dose rate, since the decaborane ion B.sub.10H.sub.x.sup.+
has ten boron atoms. Importantly, by significantly reducing the
electrical current carried in the ion beam (by a factor of 10 in
the case of decaborane ions) not only are beam space-charge effects
reduced, increasing beam transmission, but wafer charging effects
are reduced as well. Since positive ion bombardment is known to
reduce device yields by charging the wafer, particularly damaging
sensitive gate isolation, such a reduction in electrical current
through the use of cluster ion beams is very attractive for USJ
device manufacturing, which must increasingly accommodate thinner
gate oxides and exceedingly low gate threshold voltages. Thus,
there is a critical need to solve two distinct problems facing the
semiconductor manufacturing industry today: wafer charging, and low
productivity in low-energy ion implantation. As we will show later
in this document, the present invention proposes to further
increase the benefits of cluster implantation by using
significantly larger boron hydride clusters having n>10. In
particular, we have implanted the B.sub.18H.sub.x.sup.+ ion, and
further propose to implant the B.sub.36H.sub.x.sup.+ ion, using the
solid feed material octadecaborane, or B.sub.18H.sub.22. We will
present first results showing that this technology is a significant
advance over previous efforts in boron cluster implantation.
Ion Implantation Systems
Ion implanters have historically been segmented into three basic
categories: high current, medium current, and high energy
implanters. Cluster beams are useful for high current and medium
current implantation processes. In particular, today's high current
implanters are primarily used to form the low energy, high dose
regions of the transistor such as drain structures and doping of
the polysilicon gates. They are typically batch implanters, i.e.,
processing many wafers mounted on a spinning disk, the ion beam
remaining stationary. High current transport systems tend to be
simpler than medium current transport systems, and incorporate a
large acceptance of the ion beam. At low energies and high
currents, prior art implanters produce a beam at the substrate
which tends to be large, with a large angular divergence (e.g., a
half-angle of up to seven degrees). In contrast, medium current
implanters typically incorporate a serial (one wafer at a time)
process chamber, which offers a high tilt capability (e.g., up to
60 degrees from the substrate normal). The ion beam is typically
electromagnetically or electrodynamically scanned across the wafer
at a high frequency, up to about 2 kiloHertz in one dimension
(e.g., laterally) and mechanically scanned at a low frequency of
less than 1 Hertz in an orthogonal direction (e.g., vertically), to
obtain a real coverage and provide dose uniformity over the
substrate. Process requirements for medium current implants are
more complex than those for high current implants. In order to meet
typical commercial implant dose uniformity and repeatability
requirements of a variance of only a few percent, the ion beam must
possess excellent angular and spatial uniformity (angular
uniformity of beam on wafer of .ltoreq.1 deg, for example). Because
of these requirements, medium current beam lines are engineered to
give superior beam control at the expense of reduced acceptance.
That is, the transmission efficiency of the ions through the
implanter is limited by the emittance of the ion beam. Presently,
the generation of higher current (about 1 mA) ion beams at low
(<10 keV) energy is problematic in serial implanters, such that
wafer throughput is unacceptably low for certain lower energy
implants (for example, in the creation of source and drain
structures in leading edge CMOS processes). Similar transport
problems also exist for batch implanters (processing many wafers
mounted on a spinning disk) at the low beam energies of <5 keV
per ion.
While it is possible to design beam transport optics which are
nearly aberration-free, the ion beam characteristics (spatial
extent, spatial uniformity, angular divergence and angular
uniformity) are nonetheless largely determined by the emittance
properties of the ion source itself (i.e., the beam properties at
ion extraction which determine the extent to which the implanter
optics can focus and control the beam as emitted from the ion
source). The use of cluster beams instead of monomer beams can
significantly enhance the emittance of an ion beam by raising the
beam transport energy and reducing the electrical current carried
by the beam. However, prior art ion sources for ion implantation
are not effective at producing or preserving ionized clusters of
the required N- and P-type dopants. Thus, there is a need for
cluster ion and cluster ion source technology in order to provide a
better-focused, more collimated and more tightly controlled ion
beam on target, and in addition to provide higher effective dose
rates and higher throughputs in semiconductor manufacturing.
An alternative approach to beam line ion implantation for the
doping of semiconductors is so-called "plasma immersion". This
technique is known by several other names in the semiconductor
industry, such as PLAD (PLAsma Doping), PPLAD (Pulsed PLAsma
Doping, and PI.sup.3 (Plasma Immersion Ion Implantation). Doping
using these techniques requires striking a plasma in a large vacuum
vessel that has been evacuated and then backfilled with a gas
containing the dopant of choice such as boron trifluoride,
diborane, arsine, or phosphine. The plasma by definition has
positive ions, negative ions and electrons in it. The target is
then biased negatively thus causing the positive ions in the plasma
to be accelerated toward the target. The energy of the ions is
described by the equation U=QV, where U is the kinetic energy of
the ions, Q is the charge on the ion, and V is the bias on the
wafer. With this technique there is no mass analysis. All positive
ions in the plasma are accelerated and implanted into the wafer.
Therefore extremely clean plasma must be generated. With this
technique of doping a plasma of diborane, phosphine or arsine gas
is formed, followed by the application of a negative bias on the
wafer. The bias can be constant in time, time-varying, or pulsed.
Dose can be parametrically controlled by knowing the relationship
between pressure of the vapor in the vessel, the temperature, the
magnitude of the biasing and the duty cycle of the bias voltage and
the ion arrival rate on the target. It is also possible to directly
measure the current on the target. While Plasma Doping is
considered a new technology in development, it is attractive since
it has the potential to reduce the per wafer cost of performing low
energy, high dose implants, particularly for large format (e.g.,
300 mm) wafers. In general, the wafer throughput of such a system
is limited by wafer handling time, which includes evacuating the
process chamber and purging and re-introducing the process gas each
time a wafer or wafer batch is loader into the process chamber.
This requirement has reduced the throughput of Plasma Doping
systems to about 100 wafers per hour (WPH), well below the maximum
mechanical handling capability of beamline ion implantation
systems, which can process over 200 WPH.
Negative Ion Implantation
It has recently been recognized (see, for example, Junzo Ishikawa
et al. "Negative-Ion Implantation Technique", Nuclear Instruments
and Methods in Physics Research B 96 (1995) 7-12.) that implanting
negative ions offers advantages over implanting positive ions. One
very important advantage of negative ion implantation is to reduce
the ion implantation-induced surface charging of VLSI devices in
CMOS manufacturing. In general, the implantation of high currents
(on the order of 1 mA or greater) of positive ions creates a
positive potential on the gate oxides and other components of the
semiconductor device which can easily exceed gate oxide damage
thresholds. When a positive ion impacts the surface of a
semiconductor device, it not only deposits a net positive charge,
but liberates secondary electrons at the same time, multiplying the
charging effect. Thus, equipment vendors of ion implantation
systems have developed sophisticated charge control devices,
so-called electron flood guns, to introduce low-energy electrons
into the positively-charged ion beam and onto the surface of the
device wafers during the implantation process. Such electron flood
systems introduce additional variables into the manufacturing
process, and cannot completely eliminate yield losses due to
surface charging. As semiconductor devices become smaller and
smaller, transistor operating voltages and gate oxide thicknesses
become smaller as well, reducing the damage thresholds in
semiconductor device manufacturing, further reducing yield. Hence,
negative ion implantation potentially offers a substantial
improvement in yield over conventional positive ion implantation
for many leading-edge processes. Unfortunately, this technology is
not yet commercially available, and indeed negative ion
implantation has not to the author's knowledge been used to
fabricate integrated circuits, even in research and
development.
SUMMARY OF THE INVENTION
An object of this invention is to provide a method of manufacturing
a semiconductor device, this method being capable of forming
ultra-shallow impurity-doped regions of P-type (i.e., acceptor)
conductivity in a semiconductor substrate, and furthermore to do so
with high productivity.
Another object of this invention is to provide a method of
manufacturing a semiconductor device, this method being capable of
forming ultra-shallow impurity-doped regions of P-type (i.e.,
acceptor) conductivity in a semiconductor substrate using ionized
clusters of the form B.sub.nH.sub.x.sup.+ and B.sub.nH.sub.x.sup.-
where 10<n<100 and 0.ltoreq.x.ltoreq.n+4.
A further object of this invention is to provide a method of
manufacturing a semiconductor device by implanting ionized
molecules of octadecaborane, B.sub.18H.sub.22, of the form
B.sub.18H.sub.x.sup.+ or B.sub.18H.sub.x.sup.-, where x is an
integer less than or equal to 22.
A still further object of this invention is to provide for an ion
implantation system for manufacturing semiconductor devices, which
has been designed to form ultra shallow impurity-doped regions of
either N or P conductivity type in a semiconductor substrate
through the use of cluster ions.
According to one aspect of this invention, there is provided a
method of implanting cluster ions comprising the steps of:
providing a supply of molecules which each contain a plurality of
dopant atoms into an ionization chamber, ionizing said molecules
into dopant cluster ions, extracting and accelerating the dopant
cluster ions with an electric field, selecting the desired cluster
ions by mass analysis, modifying the final implant energy of the
cluster ion through post-analysis ion optics, and implanting the
dopant cluster ions into a semiconductor substrate.
An object of this invention is to provide a method that allows the
semiconductor device manufacturer to ameliorate the difficulties in
extracting low energy ion beams by implanting a cluster of n dopant
atoms (n=18 in the case of B.sub.18H.sub.x.sup.+) rather than
implanting a single atom at a time. The cluster ion implant
approach provides the equivalent of a much lower energy monatomic
implant since each atom of the cluster is implanted with an energy
of approximately E/n. Thus, the implanter is operated at an
extraction voltage approximately n times higher than the required
implant energy, which enables higher ion beam current, particularly
at the low implantation energies required by USJ formation. In
addition, each milliamp of cluster current provides the equivalent
of 18 mA of monomer boron. Considering the ion extraction stage,
the relative improvement in transport efficiency enabled by cluster
ion implant can be quantified by evaluating the Child-Langmuir
limit. It is recognized that this limit can be approximated by:
J.sub.max=1.72(Q/A).sup.1/2V.sup.3/2d.sup.-2, (1) where J.sub.max
is in mA/cm.sup.2, Q is the ion charge state, A is the ion mass in
AMU, V is the extraction voltage in kV, and d is the gap width in
cm. FIG. 2 is a graph of equation (1) for the case of
.sup.11B.sup.+ with d=1.27 cm, where the average mass of the
extracted beam is assumed to be 15 AMU. In practice, the extraction
optics used by many ion implanters can be made to approach this
limit. By extension of equation (1), the following figure of merit,
.DELTA., can be defined to quantify the increase in throughput, or
implanted dose rate, for a cluster ion implant relative to
monatomic implantation:
.DELTA.=n(U.sub.n/U.sub.1).sup.3/2(m.sub.n/m.sub.1).sup.-1/2.
(2)
Here, .DELTA. is the relative improvement in dose rate (atoms/sec)
achieved by implanting a cluster with n atoms of the dopant of
interest at an energy U.sub.n relative to the single atom implant
of an atom of mass m.sub.1 at energy U.sub.1, where U.sub.i=eV. In
the case where U.sub.n is adjusted to give the same dopant
implantation depth as the monatomic (n=1) case, equation (2)
reduces to: .DELTA.=n.sup.2. (3)
Thus, the implantation of a cluster of n dopant atoms has the
potential to provide a dose rate n.sup.2 higher than the
conventional implant of single atoms. In the case of
B.sub.18H.sub.x, this maximum dose rate improvement is more than
300. The use of cluster ions for ion implant clearly addresses the
transport of low energy (particularly sub-keV) ion beams. It is to
be noted that the cluster ion implant process only requires one
electrical charge per cluster, rather than having every dopant atom
carrying one electrical charge, as in the conventional case. The
transport efficiency (beam transmission) is thus improved, since
the dispersive Coulomb forces are reduced with a reduction in
charge density. Importantly, this feature enables reduced wafer
charging, since for a given dose rate, the electrical beam current
incident on the wafer is dramatically reduced. Also, since the
present invention produces copious amounts of negative ions of
boron hydrides, such as B.sub.18H.sub.x.sup.-, it enables the
commercialization of negative ion implantation at high dose rates.
Since negative ion implantation produces less wafer charging than
positive ion implantation, and since these electrical currents are
also much reduced through the use of clusters, yield loss due to
wafer charging can be further reduced. Thus, implanting with
clusters of n dopant atoms rather than with single atoms
ameliorates basic transport problems in low energy ion implantation
and enables a dramatically more productive process.
Enablement of this method requires the formation of the cluster
ions. The prior art ion sources used in commercial ion implanters
produce only a small fraction of primarily lower-order (e.g., n=2)
clusters relative to their production of monomers, and hence these
implanters cannot effectively realize the low energy cluster beam
implantation advantages listed above. Indeed, the intense plasmas
provided by many conventional ion sources rather dissociate
molecules and clusters into their component elements. The novel ion
source described herein produces cluster ions in abundance due to
its use of a "soft" ionization process, namely electron-impact
ionization. The ion source of the present invention is designed
expressly for the purpose of producing and preserving dopant
cluster ions. Instead of striking an arc discharge plasma to create
ions, the ion source of the present invention uses electron-impact
ionization of the process gas by electrons injected in the form of
one or more focused electron beams.
DESCRIPTION OF THE DRAWINGS
These and other advantages of the present invention will be readily
understood with reference to the following specification and
attached drawing wherein:
FIG. 1A is a schematic diagram of an exemplary high-current cluster
ion implantation system in accordance with the present
invention.
FIG. 1B is a schematic diagram of the accel-decel electrode used in
the implantation system of FIG. 1A.
FIG. 1C is an alternative embodiment of a high-current cluster ion
implantation system in accordance with the present invention.
FIG. 1D is yet another alternative embodiment of a high-current
cluster ion implantation system in accordance with the present
invention.
FIG. 1E is a schematic diagram of an exemplary medium-current
cluster ion implantation system in accordance with the present
invention.
FIG. 2 is a graphical diagram illustrating maximum .sup.11B.sup.+
beam current vs. extraction energy according to the Child-Langmuir
Law of equation (1).
FIG. 3 is a perspective view of an ion source in accordance with
the present invention, shown in cutaway to expose internal
components.
FIG. 4A is a side view of a portion of one embodiment of the ion
source shown in FIG. 3, shown in cutaway with the electron beam and
magnetic fields shown superimposed thereupon.
FIG. 4B is similar to FIG. 4A but illustrates an alternative
configuration with two electron beam sources.
FIG. 5A is a perspective diagram of the cluster ion source of FIG.
3, showing details of the ionization region.
FIG. 5B is similar to FIG. 5A but illustrates an alternative
configuration with two electron beam sources.
FIG. 5C is a simplified top view of the electron beam forming
region of the ion source illustrated in FIG. 5B.
FIG. 6 is a diagram of the 3-zone temperature control system used
in the ion source of the present invention.
FIG. 7A is a perspective view of the magnetic yoke assembly,
illustrating the magnetic circuit which includes permanent
magnets.
FIG. 7B is a perspective view of the magnetic yoke assembly
integrated into the ionization chamber of the ion source of the
present invention.
FIG. 7C is an illustration of the magnetic flux through a
cross-section of the magnetic yoke assembly in the xy plane.
FIG. 7D is a perspective view of an alternative embodiment of the
magnetic yoke assembly illustrated in FIG. 7A, which includes an
electromagnet.
FIG. 7E is similar to FIG. 7B except that it relates to the
embodiment illustrated in FIG. 7D.
FIG. 7F is an illustration of the magnetic flux through a
cross-section of the magnetic yoke assembly depicted in FIG. 7E, in
the yz plane.
FIG. 7G is similar to FIG. 7F except that it illustrates magnetic
flux in the xz plane.
FIG. 7H depicts the ion source of the present invention with a
high-permeability magnetic shield between the yoke assembly of FIG.
7B and the electron gun.
FIG. 8A is a graphical illustration of octadecaborane beam current
and vapor pressure, versus vaporizer temperature, using the ion
source of the present invention.
FIG. 8B is a ball-and-stick model of the B.sub.18H.sub.22
molecule.
FIG. 9 is a graphical illustration of the positive ion mass
spectrum of B.sub.18H.sub.22 generated with the ion source of the
present invention, collected at high mass resolution.
FIG. 10 is a graphical illustration of the negative ion mass
spectrum of B.sub.18H.sub.22 overlaid with a positive ion mass
spectrum of B.sub.18H.sub.22, both collected at high mass
resolution, and generated with the ion source of the present
invention.
FIG. 11A is a graphical illustration of the positive ion mass
spectrum of B.sub.18H.sub.22 generated with the ion source of the
present invention, collected at low mass resolution.
FIG. 11B is a graphical illustration of the positive mass spectrum
of B.sub.18H.sub.22 generated with the ion source of the present
invention, collected at highest mass resolution and with an
expanded horizontal scale, so that individual ion masses can be
resolved.
FIG. 12 is a graphical illustration of B.sub.18H.sub.x.sup.+ beam
current as a function of beam extraction energy, measured near the
wafer position by a cluster ion implantation system of the present
invention.
FIG. 13 is a graphical illustration of the data of FIG. 12
converted to boron dose rate (using B.sub.18H.sub.x.sup.+
implantation) as a function of boron implant energy, using a
cluster ion implantation system of the present invention.
FIG. 14 is a diagram of a CMOS fabrication sequence during
formation of the NMOS drain extension.
FIG. 15 is a diagram of a CMOS fabrication sequence during
formation of the PMOS drain extension.
FIG. 16 is a diagram of a semiconductor substrate in the process of
manufacturing a NMOS semiconductor device, at the step of N-type
drain extension implant.
FIG. 17 is a diagram of a semiconductor substrate in the process of
manufacturing a NMOS semiconductor device, at the step of the
source/drain implant.
FIG. 18 is a diagram of a semiconductor substrate in the process of
manufacturing an PMOS semiconductor device, at the step of P-type
drain extension implant.
FIG. 19 is a diagram of a semiconductor substrate in the process of
manufacturing a PMOS semiconductor device, at the step of the
source/drain implant.
FIG. 20 is a graphical illustration of as-implanted SIMS profiles
of boron concentrations from a 20 keV B.sub.18H.sub.x.sup.+ ion
beam implanted into a silicon wafer by a cluster ion implantation
system of the present invention.
FIG. 21 is a graphical illustration of the ionization cross-section
a as a function of electron energy T for ammonia (NH.sub.3).
DETAILED DESCRIPTION
Cluster Ion Implantation System
FIG. 1A is a schematic diagram of a cluster ion implantation system
of the high current type in accordance with the present invention.
Configurations other than that shown in FIG. 1A are possible. In
general, the electrostatic optics of ion implanters employ slots
(apertures displaying a large aspect ratio in one dimension)
embedded in electrically conductive plates held at different
potentials, which tend to produce ribbon beams, i.e., beams which
are extended in one dimension. This approach has proven effective
in reducing space-charge forces, and simplifies the ion optics by
allowing the separation of focusing elements in the dispersive
(short axis) and non-dispersive (long axis) directions. The cluster
ion source 10 of the present invention is coupled with an
extraction electrode 220 to create an ion beam 200 which contains
cluster ions, such as B.sub.18H.sub.x.sup.+ or As.sub.4.sup.+. The
ions are extracted from an elongated slot in ion source 10, called
the ion extraction aperture, by an extraction electrode 220, which
also incorporates slot lenses of somewhat larger dimension than
those of the ion extraction aperture; typical dimensions of the ion
extraction aperture may be, for example, 50 mm tall by 8 mm wide,
but other dimensions are possible. The electrode is an accel-decel
electrode in a tetrode configuration, i.e., the electrode extracts
ions from the ion source at a higher energy and then decelerates
them prior to their exiting the electrode.
A schematic diagram of the accel-decel electrode is shown in FIG.
1B. It is comprised of suppression plate 300 biased by power supply
Vs, extraction plate 302 biased by power supply Vf, and ground
plate 304, which is at implanter terminal ground (not necessarily
earth ground in a decel machine). Ion extraction aperture plate 80
is held unipotential with ionization chamber 44 of ion source 10,
which is held at ion source potential by power supply Va. For the
production of positive ions, Va>0, Vf<0, and Vs<0. For
production of negative ions, Va<0, Vf=0, and Vs>0. For
example, to produce 20 keV positive ions, typical voltages would be
Va=20 kV, Vs=-5 kV, Vf=-15 kV. Note that this means that the actual
voltages of the various plates are: extraction aperture plate 80=20
kV, suppression plate 300=-20 kV, extraction plate 302=-15 kV,
ground plate 304=0V. For producing negative ions, the power supply
voltages are reversed. By using bipolar power supplies, either
negative or positive ions may be produced by the novel implanter
designs of FIGS. 1A, 1C, 1D and 1E. Thus, ions are extracted at
higher energy from the ion source, and are decelerated upon leaving
the ground plate 304, enabling higher extracted currents and
improved focusing and transmission of the resultant ion beam
200.
The ion beam 200 (FIG. 1A) typically contains ions of many
different masses, i.e., all of the ion species of a given charge
polarity created in the ion source 210. The ion beam 200 then
enters an analyzer magnet 230. The analyzer magnet 230 creates a
dipole magnetic field within the ion beam transport path as a
function of the current in the magnet coils; the direction of the
magnetic field is shown as normal to the plane of FIG. 1A, which is
also along the non-dispersive axis of the one-dimensional optics.
The analyzer magnet 230 is also a focusing element which forms a
real image of the ion extraction aperture (i.e., the optical
"object" or source of ions) at the location of the mass resolving
aperture 270. Thus, mass resolving aperture 270 has the form of a
slot of similar aspect ratio but somewhat larger dimension than the
ion extraction aperture. In one embodiment, the width of resolving
aperture 270 is continuously variable to allow selection of the
mass resolution of the implanter. This feature is important for
maximizing delivered beam current of boron hydride cluster ions,
which display a number of ion states separated by one AMU, as for
example is illustrated in FIG. 11A. A primary function of the
analyzer magnet 230 is to spatially separate, or disperse, the ion
beam into a set of constituent beamlets by bending the ion beam in
an arc whose radius depends on the mass-to-charge ratio of the
discrete ions. Such an arc is shown in FIG. 1A as a beam component
240, the selected ion beam. The analyzer magnet 230 bends a given
beam along a radius given by Equation (4) below:
R=(2mU).sup.1/2/qB, (4) where R is the bending radius, B is the
magnetic flux density, m is the ion mass, U is the ion kinetic
energy and q is the ion charge state.
The selected ion beam is comprised of ions of a narrow range of
mass-energy product only, such that the bending radius of the ion
beam by the magnet sends that beam through mass resolving aperture
270. The components of the beam that are not selected do not pass
through the mass-resolving aperture 270, but are intercepted
elsewhere. For beams with smaller mass-to-charge ratios m/q 250
than the selected beam 240, for example comprised of hydrogen ions
having a mass of 1 or 2 AMU, the magnetic field induces a smaller
bending radius and the beam intercepts the inner radius wall 300 of
the magnet vacuum chamber, or elsewhere upstream of the mass
resolving aperture. For beams with larger mass-to-charge ratios 260
than the selected beam 240, the magnetic field induces a larger
bending radius, and the beam strikes the outer radius wall 290 of
the magnet chamber, or elsewhere upstream of the mass resolving
aperture. As is well established in the art, the combination of
analyzer magnet 230 and mass resolving aperture 270 form a mass
analysis system which selects the ion beam 240 from the
multi-species beam 200 extracted from the ion source 10. The
selected beam 240 then passes through a post-analysis
acceleration/deceleration electrode 310. This stage 310 can adjust
the beam energy to the desired final energy value required for the
specific implantation process. For example, in low-energy,
high-dose process higher currents can be obtained if the ion beam
is formed and transported at a higher energy and then decelerated
to the desired, lower implant ion energy prior to reaching the
wafer. The post-analysis acceleration/deceleration lens 310 is an
electrostatic lens similar in construction to decel electrode 220.
To produce low-energy positive ion beams, the front portion of the
implanter is enclosed by terminal enclosure 208 and floated below
earth ground. A grounded Faraday cage 205 surrounds the enclosure
208 for safety reasons. Thus, the ion beam can be transported and
mass-analyzed at higher energies, and decelerated prior to reaching
the workpiece. Since decel electrode 300 is a strong-focusing
optic, dual quadrupoles 320 refocus ion beam 240 to reduce angular
divergence and spatial extent. In order to prevent ions which have
undergone charge-exchange or neutralization reactions between the
resolving aperture and the substrate 312 (and therefore do not
possess the correct energy) from propagating to substrate 312, a
neutral beam filter 310a (or "energy filter") is incorporated
within this beam path. For example, the neutral beam filter 310a
shown incorporates a "dogleg" or small-angle deflection in the beam
path which the selected ion beam 240 is constrained to follow
through an applied DC electromagnetic field; beam components which
have become electrically neutral or multiply-charged, however,
would necessarily not follow this path. Thus, only the ion of
interest and with the correct ion energy is passed downstream of
the exit aperture 314 of the filter 310a.
Once the beam is shaped by a quadrupole pair 320 and filtered by a
neutral beam filter 310a, the ion beam 240 enters the wafer process
chamber 330, also held in a high vacuum environment, where it
strikes the substrate 312 which is mounted on a spinning disk 315.
Various materials for the substrate are suitable with the present
invention, such as silicon, silicon-on-insulator strained
superlattice substrate and a silicon germanium (SiGe) strained
superlattice substrate. Many substrates may be mounted on the disk
so that many substrates may be implanted simultaneously, i.e., in
batch mode. In a batch system, spinning of the disk provides
mechanical scanning in the radial direction, and either vertical or
horizontal scanning of the spinning disk is also effected at the
same time, the ion beam remaining stationary.
Alternative embodiments of high-current implanters are illustrated
in FIG. 1C and FIG. 1D. In particular, FIG. 1C illustrates an
accel-decel implanter similar to that described in FIG. 1A, except
that the beam line has been significantly shortened by removal of
dual quadrupoles 320 and neutral beam filter 310a. This
configuration results in better beam transmission through the
implanter, and provides for higher beam currents on substrate
312.
FIG. 1D illustrates a non-accel-decel implanter, i.e., in which the
vacuum system of the entire implanter is at earth ground. Thus, in
FIG. 1D the decel lens 310 and terminal enclosure 208 are deleted
relative to the embodiment shown in FIG. 1C. The method of cluster
beam implantation delivers very high effective dopant beam currents
at sub-keV energies, even without deceleration. The cluster beam
implantation system illustrated in FIG. 1D is greatly simplified
and more economical to produce. It also has a shorter beam line,
thus increasing the transmission of the beam to the substrate
312.
FIG. 1E schematically illustrates a proposed medium current
implanter which incorporates the present invention. There are many
alternative configurations from that which is shown in FIG. 1E. Ion
beams typically a few centimeters high and less than one centimeter
wide are produced in the ion source 400 extracted by the extraction
electrode 401 and transported through the analyzer magnet 402 and
mass resolving aperture 403. This produces a beam 404 of a specific
mass-energy product. Since the energy is fixed by the extraction
voltage, typically a single mass passes through the mass analyzer
and resolving aperture at a given analyzer magnet 402 field.
Equation (4) above describes this process. The boron hydride
cluster ion beam exits the mass resolving aperture and enters the
accel-decel electrode 405. This electrode is specifically designed
to either add energy to the ion beam or reduce the energy of the
ion beam. For low energy implants beam transport is enhanced by
extracting the beam at a higher energy and then reducing the energy
in the deceleration electrode. The Child-Langmuir Law, as
illustrated in FIG. 2, limits the current that can be extracted
from the ion source. The U.sup.3/2 dependence of current density
limit on energy, where U is the extraction energy, is responsible
for increased current at higher extraction energies. For higher
energy implants the accel-decel electrode is used to increase the
energy of the ion beam to an energy that is above the extraction
energy. Extraction energies are typically 20-40 keV, and can
decelerated to less than one keV or accelerated to energies as high
as 200 keV for singly charged ions, and as high as 500 keV for
multiply charged ions. After acceleration, the beam is transported
into a quadrupole lens 406 to refocus the beam after the energy is
adjusted by the accel-decel electrode. This step increases the
transmission efficiency through the rest of the implanter. If the
beam is allow to expand upon leaving the accel-decel region it will
hit the walls of beam line and cause particles to be generated by
the beam striking the wall of the beam line 408 as well as not
being available for implantation into the target. Next the beam
encounters the scanning module 407, which scans the beam in one
dimension, typically horizontally. The scan frequency is often in
the kiloHertz range. This causes the beam to have a very large
angular variation, resulting in the beam striking the target at
different angles on different parts of the target. To eliminate
this scan induced divergence the beam is directed through a beam
collimator 410. Beam collimators are either magnetic or
electrostatic and yield a wide parallel beam 409. The collimator
also removes ions from the beam which are at a different energy
than intended, due to charge-exchange reactions encountered in the
beam line. Upon exiting the collimator the beam enters the wafer
process chamber 411 and strikes the target 412. Medium current
implanters usually process one wafer at a time. This is known in
the industry as serial processing. Areal coverage of the wafer is
accomplished by translating the wafer in a direction orthogonal to
the direction of the beam sweep, for example, in the vertical
dimension. The frequency of the vertical is very slow compared to
the "fast" scan frequency, having a period of 5-10 or more seconds
per cycle. The dose (ions/cm.sup.2) on the wafer is controlled by
monitoring the beam current in a Faraday cup 413 mounted next to
the wafer. Once each scan, at the extreme end of the scan, the beam
enters the Faraday cup and is monitored. This allows the beam
current to be measured at a rate equal to the scan frequency of the
beam, for example 1000 times each second. This signal is then used
to control the vertical translation speed in the orthogonal
direction to beam scan to obtain a uniform dose across the wafer.
In addition, the serial process chamber allows for freedom to
orient the wafer relative to the ion beam. Wafers can be rotated
during the implant process, and can be by tilted to large angles,
as much as 60 degrees to the beam normal.
The use of cluster ion beams such as B.sub.18H.sub.x.sup.+ or
As.sub.4H.sub.x.sup.+ allow the beam extraction and transmission to
take place at higher energies than would be the case for monomers
such as B.sup.+ or As.sup.+. Upon striking the target, the ion
energy is partitioned by mass ratio of the individual, constituent
atoms. For B.sub.18H.sub.22 the effective boron energy is
10.8/216.4 of the beam energy, because an average boron atom has a
mass of 10.8 amu and the molecule has an average mass of 216.4 amu.
This allows the beam to be extracted and transported at 20 times
the implant energy. Additionally the dose rate is 18 times higher
than for a monomer ion. This results in higher throughput and less
charging of the wafer. Wafer charging is reduced because there is
only one charge for 18 atoms implanted into the wafer instead of
one charge for every atom implanted with a monomer beam.
Plasma Doping with Clusters
An alternative approach to beam line ion implantation for the
doping of semiconductors is so-called "plasma immersion". This
technique is known by several other names in the semiconductor
industry, such as PLAD (PLAsma Doping), PPLAD (Pulsed PLAsma
Doping, and PI.sup.3 (Plasma Immersion Ion Implantation). Doping
using these techniques requires striking a plasma in a large vacuum
vessel that has been evacuated and then backfilled with a gas
containing the dopant of choice such as boron trifluoride,
diborane, arsine, or phosphine. The plasma by definition has
positive ions, negative ions and electrons in it. The target is
then biased negatively thus causing the positive ions in the plasma
to be accelerated toward the target. The energy of the ions is
described by the equation U=QV, where U is the kinetic energy of
the ions, Q is the charge on the ion, and V is the bias on the
wafer. With this technique there is no mass analysis. All positive
ions in the plasma are accelerated and implanted into the wafer.
Therefore extremely clean plasma must be generated. With this
technique of doping a vapor of boron clusters such as
B.sub.18H.sub.22, or arsenic clusters such as As.sub.4H.sub.x can
be introduced into the vessel and a plasma ignited, followed by the
application of a negative bias on the wafer. The bias can be
constant in time, time-varying, or pulsed. The use of these
clusters will be beneficial since the ratio of dopant atoms to
hydrogen (e.g., using B.sub.18H.sub.22 versus B.sub.2H.sub.6 and
As.sub.4H.sub.x versus AsH.sub.3) is greater for hydride clusters
than for simple hydrides, and also the dose rates can be much
higher when using clusters. Dose can be parametrically controlled
by knowing the relationship between pressure of the vapor in the
vessel, the temperature, the magnitude of the biasing and the duty
cycle of the bias voltage and the ion arrival rate on the target.
It is also possible to directly measure the current on the target.
As with beam line implantation, using octadecaborane would yield an
18 times enhancement in dose rate and 20 times higher accelerating
voltages required if octadecaborane were the vapor of choice. If
As.sub.4H.sub.x were used there would be a four times dose rate
enhancement and a four times the voltage required. There would also
be reduced changing as with the beam line implants utilizing
clusters.
Cluster Ion Source
FIG. 3 is a diagram of a cluster ion source 10 and its various
components. The details of its construction, as well as its
preferred modes of operation, are disclosed in detail in
commonly-owned U.S. patent application Ser. No. 10/183,768,
"Electron Impact Ion Source", submitted Jun. 26, 2002, inventor T.
N. Horsky, herein incorporated by reference. The ion source 10 is
one embodiment of a novel electron impact ionization source. FIG. 3
is a cross-sectional schematic diagram of the source construction
which serves to clarify the functionality of the components which
make up the ion source 10. The ion source 10 is made to interface
to an evacuated vacuum chamber of an ion implanter or other process
tool by way of a mounting flange 36. Thus, the portion of the ion
source 10 to the right of flange 36, shown in FIG. 3, is at high
vacuum (pressure<1.times.10.sup.-4 Torr). Gaseous material is
introduced into ionization chamber 44 in which the gas molecules
are ionized by electron impact from electron beam 70A or 70B, which
enters the ionization chamber 44 through electron entrance aperture
71B such that electron beam 70A or 70B is aligned with ion
extraction aperture 81, and exits ionization chamber 44 through
electron exit aperture 71A. In one embodiment incorporating a
single electron gun and a beam dump, shown in FIG. 4A and FIG. 5A,
after leaving ionization chamber 44, the electron beam is stopped
by beam dump 72 located external to ionization chamber 44. Thus,
ions are created adjacent to the ion extraction aperture 81, which
appears as a slot in the ion extraction aperture plate 80. The ions
are then extracted and formed into an energetic ion beam by an
extraction electrode (not shown) located in front of the ion
extraction aperture plate 80. The ionization region is shown in
more detail in FIGS. 4A and 4B and in FIGS. 5A and 5B.
Referring now to FIG. 3, gases may be fed into the ionization
chamber 44 via a gas conduit 33. Solid feed materials can be
vaporized in a vaporizer 28, and the vapor fed into the ionization
chamber 44 through a vapor conduit 32 within the source block 35.
Solid feed material 29, located under a perforated separation
barrier 34a, is held at a uniform temperature by temperature
control of the vaporizer housing 30. Vapor 50 which accumulates in
a ballast volume 31 feeds through conduit 39 and through one or
more shutoff valves 100 and 110. The nominal pressure of vapor 50
within shutoff valve 110 is monitored by capacitance manometer
gauge 60. The vapor 50 feeds into the ionization chamber 44 through
a vapor conduit 32, located in the source block 35. Thus, both
gaseous and solid dopant-bearing materials may be ionized by this
ion source.
FIGS. 4A, 4B, 5A and 5B illustrate alternative embodiments of the
optical design of the ion source. In particular, FIGS. 4A and 5A
illustrate one embodiment of the invention incorporating a single
electron source. FIGS. 4B and 5B illustrate an alternative
embodiment incorporating dual electron sources.
Single Electron Source
In particular, FIG. 4A is a cross-sectional side view which
illustrates one embodiment of the optical design of the ion source
configuration in accordance with the present invention. In this
embodiment of the invention, an electron beam 70 is emitted from a
heated filament 110 and executes a 90 degree trajectory due to the
influence of beam steerers, for example, incorporating a static
magnetic field B 135 (in a direction normal to the plane of the
paper as indicated) into the ionization chamber 44, passing first
through base plate aperture 106 in base plate 105, and then through
electron entrance aperture 70a in ionization chamber 44. Electrons
passing all the way through ionization chamber 44 (i.e., through
electron entrance aperture 70a and electron exit aperture 71) are
intercepted by a beam dump 72. Emitter shield 102 is unipotential
with base plate 105 and provides electrostatic shielding for the
propagating electron beam 70. As electron beam 70 propagates
through the base plate aperture 106, it is decelerated prior to
entering ionization chamber 44 by the application of a voltage Va
to base plate 105 (provided by positive-going power supply 115),
and voltage Vc to the filament 135 (provided by negative-going
power supply 116), both biased relative to the ionization chamber
44. It is important to maintain an electron beam energy
significantly higher than typically desired for ionization in the
beam-forming and the transport region, i.e., outside of ionization
chamber 44. This is due to the space charge effects which severely
reduce the beam current and enlarge the electron beam diameter at
low energies. Thus, it is desired to maintain the electron beam
energy between about 1.5 keV and 5 keV in this region.
Voltages are all relative to the ionization chamber 44. For
example, if Vc=-0.5 kV and Va=1.5 kV, the energy of the electron
beam is therefore given by e(Va-Vc), where e is the electronic
charge (6.02.times.10.sup.-19 Coulombs). Thus, in this example, the
electron beam 70 is formed and deflected at an energy of 2 keV, but
upon entering electron entrance aperture 70a, it has an energy of
only 0.5 keV.
Other elements shown in FIG. 4A include an extracted ion beam 120,
a source electrostatic shield 101, and emitter shield 102. Emitter
shields 102 shields the electron beams 70 from fields associated
with the potential difference between base plate 105 and the source
shield 101, which is unipotential with ionization chamber 44. The
source shield 101 shields the ion beam 120 from fields generated by
the potential difference between base plate 105 and ionization
chamber 44, and also acts to absorb stray electrons and ions which
may otherwise impact the ion source elements. For this reason,
emitter shields 102 and the source shield 101 are constructed of
refractory metal, such as molybdenum. Alternatively, more complete
shielding of the ion beam 120 from magnetic fields B 135 and B' 119
may be accomplished by fabricating source shield 101 of a
ferromagnetic substance, such as magnetic stainless steel.
FIG. 5A is a cutaway view illustrating the mechanical detail and
which shows explicitly how the contents of FIG. 4A are incorporated
into the ion source of FIG. 3. Electrons are thermionically emitted
from filament 110 and accelerated to anode 140, forming electron
beam 70. Since electron beam 70 is generated external to the
ionization chamber, the emitter life is extended relative to known
configurations, since the emitter is in the low-pressure
environment of the implanter vacuum housing in which the ion source
resides, and since the emitter is also effectively protected from
ion bombardment.
Magnetic flux from permanent magnet 130 and magnetic pole assembly
125 is used to steer the beam by establishing a uniform magnetic
field across the air gap between the ends of the magnetic pole
assembly 125, wherein the electron beam 70 propagates. The magnetic
field B 135 and the electron beam energies of electron beam 70 are
matched such that electron beam 70 is deflected through
approximately 90 degrees, and passes into ionization chamber 44 as
shown. By deflecting electron beam 70 for example, through 90
degrees, no line of sight exists between emitter 110 and ionization
chamber 44 which contains the ions, thus preventing bombardment of
the emitters by energetic charged particles.
Since Va is positive relative to the ionization chamber 44,
electron beam 70 is decelerated as it passes through the gap
defined by base plate aperture 106 and electron entrance aperture
70a. Thus, the combination of base plate aperture 106 and electron
entrance aperture 70a and the gap between them, forms an
electrostatic lens, in this case, a decelerating lens. The use of a
decelerating lens allows the ionization energy of the electron beam
to be adjusted without substantially affecting the electron beam
generation and deflection.
The gap may be established by one or more ceramic spacers 132,
which support base plate 105 and act as a stand off from source
block 35, which is at ionization chamber potential. The ceramic
spacers 132 provide both electrical isolation and mechanical
support. Note that for clarity, the emitter shields 102 and the
source shield 101 are not shown in FIG. 5A. Also not shown is the
magnetic yoke assembly which is shown in FIG. 7A-7H.
Since the electron entrance aperture 106 can limit transmission of
electron beam 70, base plate 105 can intercept a significant
portion of the energetic electron beam 70. base plate 105 must
therefore be either actively cooled, or passively cooled. Active
cooling may be accomplished by passing liquid coolant, such as
water, through base plate 105, or forcing compressed air to flow
through said base plate 105. In an alternative embodiment, passive
cooling is accomplished by allowing base plate 105 to reach a
temperature whereby they cool through radiation to their
surroundings. This steady-state temperature depends on the
intercepted beam power, the surface area and emissivity of the base
plate, and the temperatures of surrounding components. Allowing the
base plate 105 to operate at elevated temperature, for example at
250 C, is advantageous when running condensable gases which can
form contaminating and particle-forming films on exposed cold
surfaces.
Dual Electron Source
FIG. 4B is an alternative embodiment of the optical design
illustrating a dual electron-beam ion source configuration. In this
embodiment of the invention, a pair of spatially separate electron
beams 70a and 70b are emitted from a pair of spacially separate
heated filaments 110a and 110b and execute 90 degree trajectories
due to the influence of beam steerers or static magnetic fields B
135a and 135b (in a direction normal to the plane of the paper as
indicated) into the ionization chamber 44, passing first through a
pair of base plate apertures 106a and 106b and a pair of spaced
apart base plates 105a and 105b, and then through a pair of
electron entrance apertures 71a and 71b. Electrons passing all the
way through the ionization chamber 44 (i.e., through both of the
electron entrance apertures 71a and 71b) are bent toward a pair of
emitter shields 102a and 102b by the beam steerers, or static
magnetic fields 135a and 135b. As the electron beams propagate
through the base plate apertures 106a and 106b, they are
decelerated prior to entering ionization chamber 44 by the
application of a voltage Va to the base plates 105a and 105b
(provided by positive-going power supply 115), and voltage Ve to
the filaments 135a and 135b (provided by negative-going power
supply 116). It is important to maintain electron beam energies
significantly higher than typically desired for ionization in the
beam-forming and the transport region, i.e., outside of ionization
chamber 44. This is due to the space charge effects which severely
reduce the beam current and enlarge the electron beam diameter at
low energies. Thus, it is desired to maintain the electron beam
energies between about 1.5 keV and 5 keV in this region.
Similar to the embodiment for a single electron source, the
voltages for a dual electron source are also all relative to the
ionization chamber 44. For example, if Ve=-0.5 kV and Va=1.5 kV,
the energy of the electron beam is therefore given by e(Va-Ve),
where e is the electronic charge (6.02.times.10.sup.-19 Coulombs).
Thus, in this example, the electron beam 70a, 70b is formed and
deflected at an energy of 2 keV, but upon entering electron
entrance aperture 71a, 71b it has an energy of only 0.5 keV.
The following table gives approximate values of magnetic field B
required to bend an electron beam with energy E through 90
degrees.
TABLE-US-00001 TABLE 1 Dependence of Magnetic Field Strength on
Electron Energy to Accomplish a 90 Degree Deflection in the Present
Invention Electron Energy E Magnetic Field B 1500 eV 51 G 2000 eV
59 G 2500 eV 66 G
Other elements shown in FIG. 4B include an extracted ion beam 120a,
a source electrostatic shield 101a, and a pair of emitter shields
102a and 102b. These emitter shields 102a and 102b serve two
purposes: to provide shielding from electromagnetic fields, and to
provide shielding from stray electron or ion beams. For example,
the emitter shields 102a and 102b shield the electron beams 70a and
70b from fields associated with the potential difference between
base plates 105a and 105b and the source shield 101, and also acts
as a dump for stray electron beams from the opposing electron
emitter. The source shield 101 shields the ion beam 120 from fields
generated by the potential difference between base plates 105a and
105b and the ionization chamber 44, and also acts to absorb stray
electrons and ions which would otherwise impact the ion source
elements. For this reason, both of the emitter shields 102a and
102b, as well as the source shield 101, are constructed of
refractory metal, such as molybdenum or graphite. Alternatively,
more complete shielding of ion beam 120a from the magnetic fields B
135a and 135b may be accomplished by constructing the source shield
101a of a ferromagnetic substance, such as magnetic stainless
steel.
FIG. 5B is a cutaway view illustrating the mechanical detail and
which shows explicitly how the contents of FIG. 4B are incorporated
into the ion source of FIG. 3. Electrons are thermionically emitted
from one or more of the filaments 110a and 110b and accelerated to
a pair of corresponding anodes 140a and 140b forming the electron
beams 70a and 70b. Such a configuration offers several benefits.
First, the filaments 110a and 110b can be operated separately or
together. Second, since the electron beams 70a, 70b are generated
external to the ionization chamber, the emitter life is extended
relative to known configurations, since the emitter is in the
low-pressure environment of the implanter vacuum housing in which
the ion source resides, and since the emitter is also effectively
protected from ion bombardment.
Magnetic flux from a pair of permanent magnets 130a and 130b and a
pair of magnetic pole assemblies 125a and 125b is used to form beam
steerers used to establish uniform magnetic fields across the air
gap between the ends of the magnetic pole assemblies 125a, 125b,
wherein the electron beam 70a, 70b propagates. The magnetic fields
135a and 135b and the electron beam energies of electron beams 70a
and 70b are matched such that electron beams 70a and 70b are
deflected 90 degrees, and pass into the ionization chamber 44 as
shown. By deflecting the electron beams 70a and 70b, for example,
through 90 degrees, no line of sight exists between the emitters
and the ionization chamber 44 which contains the ions, thus
preventing bombardment of the emitters by energetic charged
particles.
Since Va is positive relative to ionization chamber 44, the
electron beams 70A, 70B are decelerated as they pass through the
gap defined by base plate apertures 106a and 106b and the electron
entrance apertures 71a and 71b. Thus, the combination of base plate
aperture 106a and electron entrance aperture 71a, and baseplate
aperture 106b and electron entrance aperture 71b, and the gaps
between them, each forms an electrostatic lens, in this case, a
decelerating lens. The use of the decelerating lens allows the
ionization energy of the electron beam to be adjusted without
substantially affecting the electron beam generation and
deflection.
The gap may be established by one or more ceramic spacers 132a and
132b, which support each base plate 105a and 105b and act as a
stand off from the source block 35, which is at ionization chamber
potential. The ceramic spacers 132a and 132b provide both
electrical isolation and mechanical support. Note that for clarity,
emitter shields 102 and the source shield 101 are not shown in FIG.
3.
Since the electron entrance apertures 106a and 106b can limit
transmission of the electron beams, the baseplates 105a and 105b
can intercept a portion of the energetic electron beams 70a, 70b.
The baseplates 105a, 105b must therefore be either actively cooled,
or passively cooled. Active cooling may be accomplished by passing
liquid coolant, such as water, through the baseplates.
Alternatively, passive cooling may be accomplished by allowing the
baseplates to reach a temperature whereby they cool through
radiation to their surroundings. This steady-state temperature
depends on the intercepted beam power, the surface area and
emissivity of the baseplates, and the temperatures of surrounding
components. Allowing the baseplates 105a, 105b to operate at
elevated temperature, for example at 200 C, may be advantageous
when running condensable gases which can form contaminating and
particle-forming films on cold surfaces.
FIG. 5C shows a simplified top view of the electron beam-forming
region of the source illustrated in FIGS. 4B and 5B. The filament
110b is at potential Ve, for example, -0.5 keV with respect to
ionization chamber 44 (FIG. 3), and the anode 140b, the magnetic
pole assembly 125b, base plate 105b, and the emitter shield 102b
are all at anode potential Va, for example, 1.5 keV. Thus, the
electron beam energy is 2 keV. The electron beam 70b is deflected
by magnetic field 135b in the air gap between the poles of the
magnetic pole assembly 125b, such that electron beam 70b passes
through the base plate aperture 106b. Typical values for the base
plate apertures 106a and 106b and the electron entrance apertures
71a and 71b are all 1 cm in diameter, although larger or smaller
apertures are possible.
Ionization Probability
FIG. 21 illustrates how ionization probability depends on the
electron energy for electron impact ionization. Ammonia (NH.sub.3)
is used as an illustration. Probability is expressed as cross
section .sigma., in units of 10.sup.-16 cm.sup.2. Electron energy
(T) is in eV, i.e., electron-volts. Shown are two sets of
theoretical curves marked BEB (vertical IP) and BEB (adiabatic IP)
calculated from first principles, and two sets of experimental
data, from Djuric et al. (1981) and from Rao and Srivastava (1992).
FIG. 21 illustrates the fact that certain ranges of electron
energies produce more ionization than in other energy ranges. In
general, cross sections are highest for electron impact energies
between about 50 eV and 500 eV, peaking at about 100 eV. Thus, the
energy with which the electron beams enter the ionization chamber
44 is an important parameter which affects the operation of the ion
source of the present invention. The features shown in FIGS. 4A, 4B
and FIGS. 5A and 5B show how the present invention incorporates
electron optics which allow for broad control of electron impact
ionization energy while operating at nearly constant conditions in
the electron beam-forming and deflection regions of the ion
source.
Temperature Control
One aspect of the ion source of the present invention is user
control of the ionization chamber temperature, as well as the
temperature of the source block and valves. This feature is
advantageous when vaporizing condensable gases, preventing
significant coating of surfaces with condensed material, and
ensuring efficient transport of the vapor through conduit 39,
valves 100, 110, and vapor feed 32. The source utilizes a
combination of heating and cooling to achieve accurate control of
the source temperature. Separate temperature control is provided
for vaporizer 28, shutoff valves 100 and 110, and source block 35.
Ionization chamber 44 is passively heated, as is extraction
aperture plate 80, by interactions with electron beam 70, and
maintains stable operating temperature though thermally conductive
interfaces between source block 35 and ionization chamber 44, and
between ionization chamber 44 and extraction aperture plate 80,
such that source block temp<ionization chamber
temp<extraction aperture temp. External electronic controllers
(such as an Omron model E5CK) are used for temperature control.
Heating is provided by embedded resistive heaters, whose heating
current is controlled by the electronic controller. Cooling is
provided by a combination of convective and conductive gas cooling
methods, as further described, for example, in commonly owned PCT
application US01/18822, and in U.S. application Ser. No.
10/183,768, both herein incorporated by reference.
FIG. 6 illustrates a closed-loop control system for three
independent temperature zones, showing a block diagram of a
preferred embodiment in which three temperature zones are defined:
zone 1 for vaporizer body 30, zone 2 for isolation valves 100 and
110, and zone 3 for the source block 35. Each zone may have a
dedicated controller; for example, an Omron E5CK Digital
Controller. In the simplest case, heating elements alone are used
to actively control temperature above room ambient, for example,
between 18 C to 200 C or higher. Thus, resistive cartridge-type
heaters can be embedded into the vaporizer body 30 (heater 1) and
the source block 35 (heater 3), while the valves 100, 110 can be
wrapped with silicone strip heaters (heater 2) in which the
resistive elements are wire or foil strips. Three thermocouples
labeled TC1, TC2, and TC3 in FIG. 6 can be embedded into each of
the three components 30, 35, 100 (110) and continuously read by
each of the three dedicated temperature controllers. The
temperature controllers 1, 2, and 3 are user-programmed with a
temperature setpoint SP1, SP2, and SP3, respectively. In one
embodiment, the temperature setpoints are such that
SP3>SP2>SP1. For example, in the case where the vaporizer
temperature is desired to be at 30 C, SP2 might be 50 C and SP3 70
C. The controllers typically operate such that when the TC readback
does not match the setpoint, the controller's comparator initiates
cooling or heating as required. For example, in the case where only
heating is used to vary temperature, the comparator output is zero
unless TC1<SP1. The controllers may contain a look-up table of
output power as a nonlinear function of temperature difference
SP1-TC1, and feed the appropriate signals to the controller's
heater power supply in order to smoothly regulate temperature to
the programmed setpoint value. A typical method of varying heater
power is by pulse-width modulation of the power supply. This
technique can be used to regulate power between 1% and 100% of full
scale. Such PID controllers can typically hold temperature setpoint
to within 0.2 C.
Magnetic Yoke Assembly
In one embodiment, a uniform magnetic field B' 119 is established
within ionization chamber 44 by the incorporation of a permanent
magnetic yoke assembly 500, shown in FIG. 7A, into ionization
chamber 44. Referring now to FIG. 7A, magnetic flux is generated by
a pair of permanent magnets, for example, samarium-cobalt magnets
510a and 510b, and returned through yoke assembly 500 through the
gap between the C-shaped symmetrical pole pieces 520a and 520b. The
electron beam 70 enters through the hole 530a in the yoke 520a and
exits through the hole 530b in the yoke 520b. FIG. 7C shows how
yoke assembly 500 integrates into ionization chamber 44. In FIG.
7B, the ionization chamber 44 has a milled-out section which
receives the yoke assembly 500 and the poles 520a and 520b such
that the surface 550 of the yoke assembly 500 and the surface of
the ionization chamber 44 are flush. The interior wall of the
narrow annulus 540a and 540b (not shown), machined as part of
ionization chamber 44, defines an electron entrance aperture 70a
and an electron exit aperture 71, insuring that the ferromagnetic
material of the yoke assembly 500 is not exposed to the electron
beam, reducing any possibility of ferrous metals contamination
within the ionization volume of ionization chamber 44. FIG. 7C
shows lines of flux along a cross-section containing the xy plane
(x is horizontal, y is vertical, antiparallel to the direction of
propagation of electron beam 70 as shown in FIG. 5) of yoke
assembly 500, calculated with field modeling software. Very uniform
field lines 119 are generated within the propagation volume of
electron beam 70. B' 119 is directed parallel to incoming electron
beam 70 in order to confine electron beam 70.
A different embodiment of a magnetic yoke assembly is shown in FIG.
7D. This embodiment consists of a magnetic coil 610, an upper yoke
620a and an upper pole 630a, and an lower yoke 630a and a lower
pole 630b; a bobbin core 600 connects the upper yoke 620a and the
lower yoke 630b in a magnetic circuit which returns flux through
the vacuum gap between the upper pole 630a and the lower pole 630b.
Flux is generated by an electrical current through the coil 610
wire. The flux is carried by bobbin core 600 to the upper and lower
yokes 630a and 630b. By varying the coil current, the magnetic flux
density (i.e., the strength of the magnetic field) can in turn be
varied in the vacuum gap.
FIG. 7E shows a cutaway view (containing the Y-Z plane) of the
magnetic yoke assembly of FIG. 7D integrated into the ion source of
the present invention. The geometry of the yoke assembly as
depicted in FIG. 7E differs markedly from the yoke assembly
depicted in FIG. 7B. A significant departure from FIG. 7B lies in
the geometry of the yokes 620a and 620b, which are oriented along
the Y-direction (antiparallel to the direction of propagation of
the ion beam). The yoke assembly of FIG. 7E also utilizes a simpler
magnetic circuit, having only one pair of return yokes 620a and
630b, instead of the two pairs of return yokes in magnetic yoke
assembly 500 depicted in FIG. 7A. The coil 610 is embedded in the
source block 35 to provide heat sinking of the coil to the
temperature-controlled source block 35 (not shown in FIG. 7E).
FIG. 7F depicts the flux paths and flux density through the
magnetic yoke assembly of FIG. 7D, the leakage flux is largely
restricted to the anterior of the ion source, out of the ion beam
path, while a relatively uniform flux density is produced between
the poles 630a and 630b, wherein resides the ionization volume
containing electron beam 70. With a coil current of 3000 amp-turns,
a magnetic flux density of about 100 Gauss can be produced along
the Z-direction (a line joining the center of upper pole 630a and
lower pole 630b). A user-selectable flux density is thus produced
along Z from zero to 100 Gauss by controlling the electrical
current through the coil 610. Referring now to FIG. 7G, flux lines
in the X-Z plane, within the ionization region and parallel to the
plane containing ion extraction plate 80' and ion extraction
aperture 81', are shown. The Z-component of flux is quite uniform
in this region directly the ion extraction aperture 81'. The ion
extraction 81' aperture would be oriented along Z, in the plane of
the paper.
FIG. 7H depicts the incorporation of a high-permeability magnetic
shield 640 under the baseplate 105 of the electron gun, in order to
prevent the field produced by pole 630a from penetrating into
region 650 wherein the electron beam is guided through 90 degrees.
Without shield 640, stray magnetic fields along the vertical or
y-direction would cause unwanted deflection of the electron beam in
the lateral or x-direction, causing an error in the trajectory 660
of the electron beam prior to entering ionization chamber 44.
By incorporating the magnetic yoke assembly of FIG. 7B as shown in
FIG. 7H in the ion source of FIG. 4A, for example, it is realized
that the resulting use of a confining magnetic field helps to
counteract dispersive space-charge forces which would blow up
electron beam 70 subsequent to deceleration, i.e., as it enters
ionization chamber 44. This has the benefit of enabling higher
charge density in electron beam 70, hence higher ion density near
to the preferred ionization region adjacent to ion extraction
aperture 81, resulting in increased ion current 120. Further gains
may be realized by biasing beam dump 72 to a negative voltage Vr,
relative to ionization chamber 44, by power supply 117. For
example, if Vr.ltoreq.Vc, then a reflex mode may be established,
whereby primary electrons contained in electron beam 70 are
reflected from beam dump 72, increasing the effective path length
of the electrons. At sufficiently low electron energies, the
presence of confining field B' 119 causes the reflected electrons
to execute a helical trajectory along the direction of B'. We note
that B 135 and B' 119 are orthogonal in direction, B 135 deflecting
the electron beam 70 into ionization chamber 44 and B' 119
confining the resultant beam; therefore magnetic shield 118 is
added to the bottom of base plate 105. Magnetic shield 118 is made
of high-permeability metal so as to prevent the two fields from
mixing; this separates the path of electron beam 70 into two
regions of magnetic field; outside ionization chamber 44, and
within ionization chamber 44.
Method for Generating Boron Hydride Cluster Ions
The method herein described can be considered normal operation of
the ion source of the present invention where the only difference
from other operational modes is the user's choice of values for the
source parameters (feed material, feed gas flow rate, electron
ionization energy and current, and source component
temperature(s)). Solid octadecaborane, B.sub.18H.sub.22 may be
used, to produce boron hydride cluster ions of the form
B.sub.18H.sub.x.sup.+, by using the vaporizer and ion source
depicted in FIG. 3. Octadecaborane is a stable solid at room
temperature and has a vapor pressure of a few millitorr. In order
to generate useful mass flows of about 1 sccm of octadecaborane
vapor 32, the vaporizer 28 may be held at about 90 C. FIG. 8A
displays a plot of two variables as a function of vaporizer
temperature: vaporizer pressure on the right vertical axis, and ion
current delivered to the post-analysis Faraday cup of a
high-current implanter similar to that depicted in FIG. 1d.
Referring back to FIG. 3, the vaporizer pressure was measured by a
capacitance manometer 60 in communication with a valve 110. Typical
source operating parameters were: valve (100 and 110)
temperature=120 C, source block 35 temperature=120 C, electron
ionization energy=1 keV, electron beam current.apprxeq.70 mA. This
was achieved by setting Vc=-1 kV, Va=1.3 kV, Vr=-1 kV, and filament
emission current=160 mA.
FIG. 8b illustrates the molecular structure of B.sub.18H.sub.22,
and shows the relative positions of H atoms (light spheres) and B
atoms (dark spheres).
FIG. 9 shows an octadecaborane mass spectrum collected under
similar conditions to those used to generate FIG. 8A, in a cluster
ion implantation system similar to that disclosed in FIG. 1D. The
variable resolving aperture 270 was set to a high mass resolution,
which selected a four-AMU wide ion beam 240 to a downstream Faraday
cup. FIG. 10 shows an octadecaborane mass spectrum for both
negative and positive ions, collected under conditions similar to
those used to generate the data of FIG. 9. The polarity of all the
implanter power supplies were reversed to switch between negative
and positive ions, which were collected within a few minutes of one
another and recorded on the same plot. The B.sub.18H.sub.x.sup.+
and B.sub.18H.sub.x.sup.- peaks are at 210 AMU, suggesting a most
probable chemical formula for the ions of B.sub.18H.sub.16.sup.+
and B.sub.18H.sub.16.sup.-, respectively. FIG. 11A was collected
under conditions similar to those used to collect the data of FIG.
9, but with the resolving aperture 270 set to allow about 18 AMU to
pass downstream, allowing much higher B.sub.18H.sub.x.sup.+
currents. However, the lack of structure in the main peak attests
to reduced mass resolution. FIG. 11B is a detail collected at
highest mass resolution. With the resolving aperture set at <1
mm, only a single AMU was passed downstream to the Faraday. Thus,
individual boron hydride peaks separated by one AMU are clearly
visible. FIG. 12 shows a plot of beam current at the Faraday versus
extraction voltage without any deceleration of the ion beam,
collected at the low mass resolution of FIG. 11A. FIG. 13 shows the
data of FIG. 12 converted to atomic boron current versus effective
implant energy, as a means of comparison with monomer boron
implantation. Atomic boron current=18 X octadecaborane Faraday
current, and effective implant energy=11/210 X extraction voltage.
These currents are many times greater than currently attainable
with conventional monomer boron implantation, particularly without
ion deceleration.
In order to characterize the implantation profile of
B.sub.18H.sub.x.sup.+ for boron doping of semiconductors, a
commercial silicon wafer was dipped in HF solution to remove any
native oxide, and implanted in a cluster ion implantation system
similar to that disclosed in FIG. 1D. A boron dose of
2.times.10.sup.16 cm.sup.-2 was delivered by implanting a
B.sub.18H.sub.x.sup.+ dose of 1.1.times.10.sup.15 cm.sup.-2. The
B.sub.18H.sub.x.sup.+ ion energy was 20 keV during the implant,
resulting in an effective boron implant energy of about 1 keV per
boron atom. FIG. 20 shows the as-implanted boron profile as
determined by SIMS (Secondary Ion Mass Spectrometry). The peak of
the profile is at about 50 .ANG., which agrees well with a
projected range of 58 .ANG. predicted by TRIM calculations for a 1
keV boron implant.
Formation of N- and P-Type Shallow Junctions
An important application of this method is the use of cluster ion
implantation for the formation of N- and P-type shallow junctions
as part of a CMOS fabrication sequence. CMOS is the dominant
digital integrated circuit technology in current use and its name
denotes the formation of both N-channel and P-channel MOS
transistors (Complementary MOS: both N and P) on the same chip. The
success of CMOS is that circuit designers can make use of the
complementary nature of the opposite transistors to create a better
circuit, specifically one that draws less active power than
alternative technologies. It is noted that the N and P terminology
is based on Negative and Positive (N-type semiconductor has
negative majority carriers, and vice versa), and the N-channel and
P-channel transistors are duplicates of each other with the type
(polarity) of each region reversed. The fabrication of both types
of transistors on the same substrate requires sequentially
implanting an N-type impurity and then a P-type impurity, while
protecting the other type of devices with a shielding layer of
photoresist. It is noted that each transistor type requires regions
of both polarities to operate correctly, but the implants which
form the shallow junctions are of the same type as the transistor:
N-type shallow implants into N-channel transistors and P-type
shallow implants into P-channel transistors. An example of this
process is shown in FIGS. 14 and 15. In particular, FIG. 14
illustrates a method for forming the N-channel drain extension 89
through an N-type cluster implant 88, while FIG. 15 shows the
formation of the P-channel drain extension 90 by a P-type cluster
implant 91. It is to be noted that both N- and P-types of
transistors requires shallow junctions of similar geometries, and
thus having both N-type and P-type cluster implants is advantageous
for the formation of advanced CMOS structures.
An example of the application of this method is shown in FIG. 16
for the case of forming an NMOS transistor. This figure shows
semiconductor substrate 41 which has undergone some of the
front-end process steps of manufacturing a semiconductor device.
For example, the structure consists of a N-type semiconductor
substrate 41 processed through the P-well 43, trench isolation 42,
and gate stack formation 44, 45 steps. An exemplary process for
forming the gate stack, P-well and trench isolation is disclosed in
co-pending patent application PCT/US03/19085, filed on Jun. 18,
2003, entitled "A semiconductor Device and Method of Fabricating a
Semiconductor Device".
The P-well 43 forms a junction with the N-type substrate 41 that
provides junction isolation for the transistors in the well 43. The
trench isolation 42 provides lateral dielectric isolation between
the N- and P-wells (i.e., in the overall CMOS structure). The gate
stack is constructed, with a gate oxide layer 44 and a polysilicon
gate electrode 45, patterned to form a transistor gate stack. A
photoresist 46 is applied and patterned such that the area for NMOS
transistors is exposed, but other areas of the substrate 41 are
shielded. After the photoresist 46 is applied, the substrate 41 is
ready for the drain extension implant, which is the shallowest
doping layer required by the device fabrication process. A typical
process requirement for leading-edge devices of the 0.13 .mu.m
technology node is an arsenic implant energy of between 1 keV and 2
keV, and an arsenic dose of 5.times.10.sup.14 cm.sup.-2. The
cluster ion beam 47, As.sub.4H.sub.x.sup.+ in this case, is
directed at the semiconductor substrate, typically such that the
direction of propagation of the ion beam is normal to the
substrate, to avoid shadowing by the gate stack. The energy of the
As.sub.4H.sub.x.sup.+ cluster should be four times the desired
As.sup.+ implant energy, e.g., between 4 keV and 8 keV. The
clusters dissociate upon impact with the substrate, and the dopant
atoms come to rest in a shallow layer near the surface of the
semiconductor substrate, which forms the drain extension region 48.
We note that the same implant enters the surface layer of the gate
electrode 49, providing additional doping for the gate electrode.
The process described in FIG. 16 is thus one important application
of the proposed invention.
A further example of the application of this method is shown in
FIG. 17: the formation of the deep source/drain regions. This
figure shows the semiconductor substrate 41 of FIG. 16 after
execution of further processes steps in the fabrication of a
semiconductor device. The additional process steps include the
formation of a pad oxide 51 and the formation of spacers 52 on the
sidewalls of the gate stack. The pad oxide 51 is a thin layer of
oxide (silicon dioxide) used to protect the exposed substrate
areas, the top of the gate electrode 49 and the potentially exposed
gate dielectric edge. The pad oxide 51 is typically thermally grown
to a thickness of 5-10 nm. The spacer 52, on the other hand, is a
region of dielectric, either silicon dioxide, silicon nitride, or a
combination of these, which resides on the side of the gate stack
and serves to insulate the gate electrode. It also serves as an
alignment guide for the source/drain implant (e.g., 54), which must
be spaced back from the gate edge for the transistor to operate
properly. The spacers 52 are formed by the deposition of silicon
dioxide and/or silicon nitride layers which are then plasma etched
in a way to leave a residual layer on the side of the gate stack
while clearing the dielectrics from the source/drain region.
After etching the spacers 52, a photoresist layer 53 is applied and
patterned to expose the transistor to be implanted, an NMOS
transistor in this example. Next, the ion implant to form the
source and drain regions 55 is performed. Since this implant
requires a high dose at low energy, it is an appropriate
application of the proposed cluster implantation method. Typical
implant parameters for the 0.13 um technology node are
approximately 6 keV per arsenic atom (54) at an arsenic dose of
5.times.10.sup.15 cm.sup.-2, so it requires a 24 keV,
1.25.times.10.sup.15 cm.sup.-2 As.sub.4H.sub.x.sup.+ implant, a 12
keV, 2.5.times.10.sup.15 cm.sup.-2 As.sub.2H.sub.x.sup.+ implant,
or a 6 keV, 5.times.10.sup.15 cm.sup.-2 As.sup.+ implant. As shown
in FIG. 16, the source and drain regions 55 are formed by this
implant. These regions provide a high conductivity connection
between the circuit interconnects (to be formed later in the
process) and the intrinsic transistor defined by the drain
extension 48 in conjunction with the channel region 56 and the gate
stack 44, 45. The gate electrode 45 can be exposed to this implant
(as shown), and if so, the source/drain implant provides the
primary doping source for the gate electrode. This is shown in FIG.
17 as the poly doping layer 57.
The detailed diagrams showing the formation of the PMOS drain
extension 148 and PMOS source and drain regions 155 are shown in
FIGS. 18 and 19, respectively. The structures and processes are the
same as in FIGS. 17 and 18 with the dopant types reversed. In FIG.
18, the PMOS drain extension 148 is formed by the implantation of a
boron cluster implant 147. Typical parameters for this implant
would be an implant energy of 500 eV per boron atom with a dose of
5.times.10.sup.14 cm.sup.-2, for the 0.13 .mu.m technology node.
Thus, a B.sub.18H.sub.x.sup.+ implant at 211 AMU would be at 9.6
keV at an octadecaborane dose of 2.8.times.10.sup.13 cm.sup.-2.
FIG. 19 shows the formation of the PMOS source and drain regions
148, again by the implantation of a P-type cluster ion beam 154
such as octadecaborane. Typical parameters for this implant would
be an energy of around 2 keV per boron atom with a boron dose of
5.times.10.sup.15 cm.sup.-2 (i.e., 38.4 keV octadecaborane at
2.8.times.10.sup.14 cm.sup.-2) for the 0.13 um technology node.
In general, ion implantation alone is not sufficient for the
formation of an effective semiconductor junction: a heat treatment
is necessary to electrically activate the implanted dopants. After
implantation, the semiconductor substrate's crystal structure is
heavily damaged (substrate atoms are moved out of crystal lattice
positions), and the implanted dopants are only weakly bound to the
substrate atoms, so that the implanted layer has poor electrical
properties. A heat treatment, or anneal, at high temperature
(greater than 900 C) is typically performed to repair the
semiconductor crystal structure, and to position the dopant atoms
substitutionally, i.e., in the position of one of the substrate
atoms in the crystal structure. This substitution allows the dopant
to bond with the substrate atoms and become electrically active;
that is, to change the conductivity of the semiconductor layer.
This heat treatment works against the formation of shallow
junctions, however, because diffusion of the implanted dopant
occurs during the heat treatment. Boron diffusion during heat
treatment, in fact, is the limiting factor in achieving USJ's in
the sub-0.1 micron regime. Advanced processes have been developed
for this heat treatment to minimize the diffusion of the shallow
implanted dopants, such as the "spike anneal". The spike anneal is
a rapid thermal process wherein the residence time at the highest
temperature approaches zero: the temperature ramps up and down as
fast as possible. In this way, the high temperatures necessary to
activate the implanted dopant are reached while the diffusion of
the implanted dopants is minimized. It is anticipated that such
advanced heat treatments would be utilized in conjunction with the
present invention to maximize its benefits in the fabrication of
the completed semiconductor device.
Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
* * * * *